"I have this issue with a Microsoft keyboard on a Windows XP machine as well. It seems to be a problem with either the Microsoft IntelliType software or keyboard driver, and it seems to be just the calculator button.
One ""solution"" from the Windows 7 Taskforce website is to tap the key twice to pull the calculator to the front.
This same behavior also appears when the calculator key is set to launch another program - see Issue 302 for SpeedCrunch."